FIG. 6 shows a conventional power transmission system which is used on crawler-type vehicles such as bulldozers and which is capable of performing both linear speed change and steering. This system, known as 2-path hydrostatic transmission (referred to as 2-path HST, hereinafter) has, for the left and right crawler driving wheels 6, 7, hydraulic pumps 2, 3 driven by an engine 1, hydraulic motors 4, 5 and reduction devices 6a, 6b.
The power transmission system having such a 2-path HST, however, suffers from the following disadvantages due to the fact that all of the torque is transmitted through the HST.
(1) Power transmission efficiency is low although the operability is superior. PA1 (2) This type of power transmission system is difficult to apply to large-size vehicles because hydraulic pumps and hydraulic motors capable of transmitting large power are not readily available are or, if available, very expensive. PA1 (3) Straight running performance of the vehicle is affected by efficiencies of the hydraulic pumps and motors. PA1 (4) A complicated control is required for keeping the running course of the vehicle when the control is to be done through the control of operation of the hydraulic pumps and motors.
Accordingly, an object of the present invention is to provide a 2-path hydromechanical transmission (referred to as 2-path HMT, hereinafter) which employs an HST incorporating hydraulic pumps and motors of small capacities and which exhibit high efficiency with simple construction and ease of maneuver, as well as a control method for controlling such a 2-path HMT, thereby overcoming the above-described problems of the prior art.